Optical coherence tomography imaging systems and methods

ABSTRACT

Optical coherence tomography imaging systems and methods are disclosed. According to an aspect, an optical coherence tomography imaging system includes a scanner configured to obtain images and to convert the images to electrical signals. The system also includes a computing device comprising an OCT module configured to receive the electrical signals, to apply an OCT imaging technique, and to generate imaging data.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation patent application of U.S. Utility patentapplication Ser. No. 14/469,474, filed Aug. 26, 2014, and titled OPTICALCOHERENCE TOMOGRAPHY IMAGING SYSTEMS AND METHODS, which claims thebenefit of and priority to U.S. Provisional Patent Application No.61/869,991, filed Aug. 26, 2013 and titled OPTICAL COHERENCE TOMOGRAPHYIMAGING SYSTEMS AND METHODS, the content of which is incorporated byreference in its entirety.

TECHNICAL FIELD

The present subject matter relates to imaging. More particularly, thepresent subject matter relates to optical coherence tomography (OCT)imaging systems and methods.

BACKGROUND

OCT is an imaging modality that can be thought of as the optical analogto ultrasound. Focused light illuminates a sample and backscatteredlight is collected and by use of interferometry, depth gated as to whereit backscattered from. This allows one to see into samples in a similarfashion to ultrasound. To build up a 2 or 3 dimensional image, the lightbeam is typically scanned across the sample in one or two directions.OCT is ideally positioned for imaging samples where 1 to 5 millimetersof penetration is needed with resolution of 2 to 15 microns.

OCT is widely used in an increasing number of applications including,but not limited to, medical (e.g., ophthalmology, intravascular,oncology, dermatology, neurology, gastroenterology, ear, nose and throat(ENT)), biomedical research (developmental biology, small animalimaging, biofilm imaging, and tissue engineering), and industrial (e.g.,carbon fiber composites, art inspection, multilayer thin film thickness,plastic seal monitoring, contact lens production, and coatingqualification). Lowering the system price will both increase the usagein these areas and open up new areas for application.

The first OCT systems used a time domain architecture where depthscanning was achieved by physically moving a mirror in the referencearm. In early 2000s, the Fourier domain approach to OCT was inventedwith two implementations, spectral domain OCT where a broadband lightsource is used in conjunction with a spectrometer and swept source OCTwhere a laser is swept in wavelength and different wavelengths aresampled at different times. Fourier domain OCT architectures have a SNRadvantage over time domain ones that is proportional to the number ofpixels in the spectrometer or the number of samples in one laser sweep.FD-OCT systems have now displaced time domain systems in most clinicalapplications although there are a few industrial applications where timedomain OCT is still prevalent.

In spectral domain OCT system architecture there are four criticalcomponents that set the performance: the light source, the grating andcamera inside the spectrometer, and the scanner. Even though OCT hascommercially exploded in the last decade, it is not yet a large enoughmarket by itself to drive component development. Therefore, the advancedcomponents used in most OCT systems were originally developed for otherapplications. For example, most cameras used in OCT spectrometers areline scan cameras designed for machine vision applications. Thesecameras have very high line rates (up to 140,000 lines/second), but haveshort pixel dimensions since they are used to image items passing byquickly on conveyor belts such that the translation of the objectprovides the 2nd dimension for imaging. When used in spectrometers,these cameras are difficult to align and maintain since spectrallydispersed light forms a line that is approximately 20 mm wide by 6-7microns tall and the line scan array is 20 millimeters wide by 20microns tall. These cameras are also fairly expensive with even low endmodels costing at least $2,000.

Moving beyond the research and industrial markets, there is tremendousopportunity for low cost OCT in clinical areas, such as at the point ofcare and for clinical care in the developing world, but the regulatoryand manufacturing requirements for a clinical system require morecapital to address. Some potential clinical applications include a lowcost retinal scanner that could be widely deployed both in the UnitedStates for imaging of patients with diabetic retinopathy, glaucoma, ormacular degeneration or use in the developing world for retinalscreening of newborns and infants. Regulatory overhead and moresophisticated software may increase the cost of a clinical unit, but itcould still be greater than $12,000.

For at least the aforementioned reasons, there is a continuing need forlow cost OCT imaging systems and techniques that provide high qualityimages.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

Disclosed herein are low cost OCT imaging systems for imaging research,industrial, and clinical samples. In one or more aspects, advantage istaken of several components that are either new or are coming down incost because they are used in high volumes in other applications. Inparticular, the CMOS line scan array can be used in barcode reading,fingerprinting and edge detection; all high volume, but low costapplications. Likewise, a steerable liquid lenses can be used forscanning and have been designed for use in cell phone cameras as amotion compensation mechanism. As the user moves the camera up and down(or side to side), the lens may steer so that the sensor is stilllooking at the same target.

Further, the present subject matter covers a very low cost spectraldomain optical coherence tomography system. The target system price willbe greater than 3 times less expensive than any currently availableresearch OCT system while providing system performance comparable toother entry level OCT imaging systems. By breaking the $10,000 systembarrier, a tool is provided that will be accessible to most researchlabs and will not be confined to shared resource labs. Oneimplementation can target research and industrial applications where lowcost is an advantage and system sales do not require regulatoryapproval.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofvarious embodiments, is better understood when read in conjunction withthe appended drawings. For the purposes of illustration, there is shownin the drawings exemplary embodiments; however, the presently disclosedsubject matter is not limited to the specific methods andinstrumentalities disclosed. In the drawings:

FIG. 1 is a perspective view of a OCT imaging system including acomputer, an OCT engine, and a wand scanner according to embodiments ofthe present subject matter;

FIG. 2 is a block diagram of an example architecture for the OCT imagingsystem 100 in accordance with embodiments of the present subject matter;

FIG. 3 is a block diagram of an example architecture for another OCTimaging system in accordance with embodiments of the present subjectmatter;

FIG. 4 is a perspective view of an OCT imaging system in accordance withembodiments of the present subject matter;

FIG. 5 is a block diagram of another example architecture for an OCTsystem in accordance with embodiments of the present subject matter;

FIG. 6 is a diagram of an example wand scanner in accordance withembodiments of the present subject matter;

FIG. 7 is a diagram of an example of a wand scanner in which a singlesteerable lens is used in conjunction with a collimating lens to scanthe beam across the sample in accordance with embodiments of the presentsubject matter;

FIG. 8 is a diagram of a wand scanner in which a lens is added near thesample to help collimated the scanning beam as it moves across thesample in accordance with embodiment of the present subject matter;

FIG. 9 is a diagram of an example wand scanner in which the polarizationmatching occurs in the OCT engine, via manual paddles, motorized paddlesor other method;

FIG. 10 is a diagram of an example wand scanner in which a camera hasbeen added to the scanner to allow visual imaging of the sample whileOCT images are being taken;

FIG. 11 is a cross-sectional perspective view of the interior of anexample spectrometer in accordance with embodiments of the presentsubject matter;

FIG. 12 is a cross-sectional top view of the spectrometer shown in FIG.11;

FIG. 13 is a schematic view of another example spectrometer with loopdesign in accordance with embodiments of the present subject matter;

FIG. 14 is a perspective, cross-section and exploded view of anotherexample spectrometer plus input optics for a spectral domainimplementation of a fiber bundle OCT system in accordance withembodiments of the present subject matter;

FIG. 15 is a block diagram for a version of the fiber bundle OCT systemwith the reference arm in the OCT engine;

FIG. 16 is a block diagram for an example fiber bundle OCT system withthe reference arm in fiber probe in accordance with embodiments of thepresent subject matter; and

FIGS. 17A-17E illustrate diagrams of example probe tip geometries andoptics in accordance with embodiments of the present subject matter.

DETAILED DESCRIPTION

The presently disclosed subject matter is described with specificity tomeet statutory requirements. However, the description itself is notintended to limit the scope of this patent. Rather, the inventors havecontemplated that the claimed subject matter might also be embodied inother ways, to include different steps or elements similar to the onesdescribed in this document, in conjunction with other present or futuretechnologies. Moreover, although the term “step” may be used herein toconnote different aspects of methods employed, the term should not beinterpreted as implying any particular order among or between varioussteps herein disclosed unless and except when the order of individualsteps is explicitly described. FIG. 1 is a perspective view of a OCTimaging system 100 including a computer 102, an OCT engine 104, and awand scanner 106 according to embodiments of the present subject matter.Referring to FIG. 1, the computer 102 may be any suitable computingdevice having one or more processors and memory. The memory may beconfigured with instructions for implementation by the processor(s) forcarrying out the function described herein. As an alternative, forexample, the computing device may include suitable hardware, software,firmware, or combinations thereof for carrying out the functionsdescribed herein.

The computer 102 may be communicatively connected to the OCT engine 104.The OCT engine 104 may be communicatively connected to the wand scanner106. The OCT engine 104 and the scanner 106 may be any suitable type ofequipment configured to obtain images and to convert the images toelectrical signals representative of the images. The wand scanner 106may be shaped and sized for holding by a person. For example, the wandscanner 106 may be configured as any suitable handheld device. Thecomputer 102 may be configured to receive the electrical signals, toapply an OCT imaging technique, and to generate imaging data. Thecomputer 102 may display images corresponding to the imaging data on itsdisplay.

The computer 102, OCT engine 104, and scanner 106 may suitablycommunicate with each other. In this example, the components communicatewith each other via a suitable wireless communications technique such asBluetooth communications. Alternatively, the components may communicatevia a wired communications technique.

System Architecture

FIG. 2 illustrates a block diagram of an example architecture for theOCT imaging system 100 in accordance with embodiments of the presentsubject matter. Referring to FIG. 2, the system 100 may include threemajor subsystems: a computer 102 with software; an OCT engine 104, and ascanner 106. The computer 102 may be any suitable type of computer. Forexample, it may be a computer using the Android operating system, awidely used and available operating system from multiple hardwaremanufacturers including Samsung, HP, and others. The OCT software maycontrol the rest of the OCT system, collect raw data from thespectrometer and transform it into imaging data, store and retrieve OCTimages and/or raw data, and provide a user interface for the system.Other operating systems or computers may be used including embeddedprocessor, systems on modules or system on chips.

The OCT engine 104 may contain a light source 200, a spectrometer 202and interferometer, and an electronics control board 206. The OCT engine104 may also include a fiber splitter 208. The light source 200 may be asuperluminescent diode (SLD) or other broadband light source. Thespectrometer 202 may be the low cost spectrometer as described infurther detail herein or another suitable spectrometer. Theinterferometer may be a fiber coupler or other suitable implementation.

The scanner 106 may be a wand scanner or another suitable scanner. Thewand scanner is described in further detail herein or may be anothersuitable scanner. The scanner 106 may include one or more steerableliquid lens. The scanner 106 may contain some controls for path lengthmatching, polarization matching or control and power control.

As embedded processors have become more powerful, it is now possible toreplace the external computer with an embedded processor inside of theOCT engine 104. For example, FIG. 3 illustrates a block diagram of anexample architecture for another OCT imaging system 100 in accordancewith embodiments of the present subject matter. Referring to FIG. 3, asystem on module (SoM) 300 is used in place of a computer. FIG. 4illustrates a perspective view of an OCT imaging system 100 inaccordance with embodiments of the present subject matter. One or moreelectronics boards may be configured to implement functions ofcontrolling the scanner, the light source 200, the spectrometer 202,collecting data from the spectrometer 202, and processing that data togenerate OCT images. Example embedded processors include, but are notlimited to, the Variscite VAR-SOM-SD600 CPU, which uses the QualcommSnapdragon 600. The system 100 may include an input/output (I/O) on theengine, for example using a touchscreen 302. In addition or as analternative, the engine may communicate with an external device viawireless communication such as Bluetooth, WI-FI, or other suitablewireless technique. As part of the system, there may be one or moreapplications that reside on a tablet computer or smartphone andcommunicate with the OCT engine. These applications may allow control ofthe engine and the scanner and may receive and display OCT images fromthe OCT engine. Further, the application may subsequently communicatewith a central repository via wireless or cellular communication totransmit and store the OCT images for subsequent imaging, analysis,display, or as part of an electronic medical record (EMR).

Note that one of the advantages of this approach is low electrical powerconsumption. By using an uncooled SLD (i.e., one without a TEC) and aCMOS linescan array, it is possible to get the power consumption of theengine below 5 W. The embedded processor can consume additional power,but the entire system could be below 10 W. This can enable the system tooperate for several hours on a laptop battery or similar battery. Inturn, the system 100 can now be used in a place where there is noavailable wall power or in an environment where it is moved from room toroom, such as a doctor's clinic, without needing to be plugged into wallpower in every room.

In OCT, there are several physical properties that need some degree ofcontrol for acceptable interferometry and hence OCT signal to noise.Examples include, but are not limited to, pathlength matching,polarization matching, and power control. For pathlength matching, thetwo arms of the interferometer may have the same optical pathlengthwithin a fraction of a millimeter in order for interference fringes tobe detected with the spectrometer. In this system, coarse pathlengthmatching (within +/−3 mm) may be achieved by controlling the length ofthe reference arm fiber during manufacture. Fine control may be achievedwithin the wand scanner by optimizing the focus of the liquid lenses.

In accordance with embodiments, one or both of the scanner and computingdevice may be configured to polarization control. For example,polarization matching may be accomplished by incorporating a plasticfilm quarter wave plate in the wand scanner that can be rotated tooptimize the OCT signal. Even though two wave plates are typicallyneeded for best optimization, a single wave plate may be used as a costsaving measure that can provide enough control to ensure that thepolarizations in the two arms are not orthogonal. Alternately,polarization matching may be accomplished by using coils of fiber thatare then rotated. Different numbers of coils, one through four,approximate different wave plates depending on the wavelength of lightand the type of fiber. Reasonable polarization control can often beobtained with two coils where the first has a single loop and the secondhas two loops of fiber.

Power control may be accomplished by setting the reflection level fromthe fiber tip in the reference arm. Initial design indicates that thefiber to air interface which will result reflection about 4% of thelight will be adequate. The light source may generate approximately 2 mWof power, giving 1 mW in the reference arm after the 50/50 splitter, 40microW returning, and 20 microW at the linescan array. Additional lossescan drop the power level close to the 10 microW estimated for fillingthe linescan array. The integration time of the linescan array may thenbe adjusted to provide fine tuning of the amount of light collected inone integration time. In particular, the integration time may beshortened so that the light does not saturated the detector above acertain level.

Polarization Sensitive System

In accordance with embodiments, OCT is polarization sensitive OCT wherethe interferometer works in both polarizations and signals are collectedand processed for both polarizations. By collecting two polarizations,it is possible to image the birefringence (or polarization dependentindex of refraction) of the sample. Additional information may beprovided about the sample such as stresses in a molded plastic part ortissue characteristics in animal or humans. Polarization sensitivespectral domain OCT may require a more sophisticated spectrometer andmay require additional optics elements in the OCT engine or wandscanner. Some optics, such as the waveplate, may be removed, since thesystem may not need to be optimized for a single polarization sincemultiple polarizations are present and provide useful imaginginformation.

The polarization sensitive spectrometer may be implemented in a varietyof ways including, but not limited to, a multilinescan array with opticsfor spatially separating the polarizations or two or more linescanarrays with optics to separate the polarizations so that onepolarization is directed to one of the linescan arrays and the otherpolarization is directed to another linescan array. The polarizationsbetween A-scans may be changed so that one A-scan has one polarizationand a subsequent A-scan has a different polarization. This may slow downthe overall A-scan rate, but that may be acceptable in a low costsystem.

The wand scanner 106 may implement PS-OCT. As an example, a relativelylow cost version of PS-OCT may be implemented that remains within thecost structure of this low cost OCT while providing additionalinformation about the sample. One potential implementation of PS-OCT isshown in FIG. 5, which illustrates a block diagram of another examplearchitecture for an OCT system 100 in accordance with embodiments of thepresent subject matter. Referring to FIG. 5, the system 100 is apolarization sensitive OCT system with a wand scanner. Here, the lowcost spectrometer has been replaced by a polarization sensitivespectrometer 202. There may be additional polarization optics betweenthe light source 200 and the fiber splitter 208 (interferometer). Thesystem 100 may include polarization optics in the OCT engine 104. Thespectrometer 202 may have sensitivity to polarization, and this may beimplemented by any suitable technique. The polarization sensitivespectrometer 202 may include an optical depolarizer, since manybroadband light sources such as SLDs are fairly polarized, depolarizingmay be useful to get the optical power closer to equal in the twopolarizations. In an example, a 50:50 power split may be providedbetween the two polarizations. Alternatively, for example, any othersuitable power split may be provided.

Swept Source Optical Coherence Tomography

Although much of the present disclosure describes spectral domain OCTsystems, several embodiments disclosed herein may be suitable with sweptsource OCT systems. In swept source OCT, a tunable laser and one or morephotodiodes can be used in the OCT engine 104 in place of the broadbandlight source 200 and the spectrometer 202. The system performance may bethe same or nearly so, but now the spectral information can be collectedin a time sequenced fashion instead of simultaneously as in spectraldomain OCT.

In particular, the steerable lens based wand scanner can work quite wellwith a swept source OCT engine. The internal engine components can bechanged out for a swept source implementation, and the wand scannerconstruction and operation can be very similar to what is describedherein. The optimization of pathlength matching, polarization matching,and power control may be similar to the spectral domain OCT approach.

Low Cost Wand Scanner Based on Steerable Liquid Lenses

OCT systems can achieve optical scanning by using either a pair of galvomirrors or a MEMS mirror to direct the beam across the sample. As analternative, liquid lenses may be utilized and such lenses have beenavailable for some time and are now produced in steerable versions.Lenses that may be used with systems described herein include, but arenot limited to, the Baltic 617 made available by Varioptic. The focusrange for these lenses is −5 diopters to +15 diopters with a tilt rangeof +/−0.6 degrees. The tangent of 0.6 degrees is 0.01 which whenmultiplied by the distance between the lenses and the sample can givehalf the lateral scan range. For a 100 millimeter spacing between thelens and the sample, the scan range can be 2 millimeters. This may betoo small for practical imaging, so two of these may be stacked togenerate a full scan range of 4 millimeters. Maximum speed for existinglenses is 10 Hz. The lenses can tilt the beam in any direction, solinear scans, circular scans, and eventually 3D scans may be executedwhile simultaneously adjusting focus. Control of the liquid lenses canbe accomplished via a scanner control cable to carry the voltages fordriving two axes on each of the two lenses where each lens requires 50mW of power for scanning. Alternately, the voltages may be generated inthe wand scanner and control signals sent over the cable via digitalcommunication. More than two steerable lenses may be used if needed. Forsteerable lenses that scan in two directions, a scan may be utilizedthat is along the diagonal and is thus longer than a scan that isaligned with one of scan axis. In the previous example, the total scanlength can be increased by a factor of square root of by using thediagonal and thus giving a total scan range of 5.6 mm.

FIG. 6 illustrates a diagram of an example wand scanner 106 inaccordance with embodiments of the present subject matter. Referring toFIG. 6, the wand scanner 106 may include and be configured to operatesteerable liquid lenses. Light may enter the scanner 106 on the rightside from an input fiber 600. A lens 602 may receive the light and maybe configured to collimate the light. The lens 602 may have a focallength of ˜16 millimeters or any other suitable length or configuration.The collimated light passes through a rotatable waveplate 604 and thenthrough a pair of steerable liquid lenses 606. The light is focused atthe distal tip 608 of the scanner 106. The scanner 106 may include aremovable tip 608 with an anti-reflection coated window on a threadedinsert to thereby allowing fine adjustment of the pathlength matchingbetween the reference arm in the OCT engine and the scanner. The liquidlenses can allow adjustment of focus over a few millimeter range. Fornon-contact imaging, the tip 608 can be completely unscrewed and fineoptimization can do accomplished using only the focus control of theliquid lenses 606. The scanner 106 can also be mounted to a translationstage that holds it vertically for scanning samples in a bench topconfiguration. Using a stand can also minimize movement between thescanner 106 and the sample for either 3D imaging or averaging of manyB-scans to produce higher SNR images, if desired.

The rotatable wave plate 604 may allow for polarization matching betweenthe reference arm in the engine and the sample arm. Since the fiber inthe engine can be fixed in place, the polarization for the reference armmay not change significantly over time. However, the fiber from theengine to the wand scanner 106 can be moved as the scanner 106 ismanipulated during use. Therefore, some form of polarizationcompensation may be needed. For good polarization matching, one may needboth a quarter wave plate and a half wave plate. In this case, perfectpolarization matching may not be achieved by the system, but instead alimited degree of control is offered. This can ensure that the deviceavoids the case where the polarizations of the sample and reference armsare orthogonal to each other and the interference fringe goes to zero. Asingle rotatable wave plate can allow the polarization of the sample armto be shifted and provide sufficient control to avoid the crossedpolarization condition. The user can rotate the wave plate 604 whilelooking at the OCT image to optimize the signal strength of the image.In an alternative example, a second wave plate can be utilized byintegration into the wand scanner 106.

The liquid lenses described herein are steerable lenses, although anysuitable type may be used. Like most new products based on newtechnologies, performance can be expected to improve over time.Improvement may occur in any of the performance attributes of the liquidlenses 606. As an example, scan range can be improved by use of theBaltic 617, which has a steerable range of +/−0.6 degrees. This mayincrease as this product is improved and other versions of thistechnology come on the market. For OCT, larger scan range is better, itwould be ideal if a single lens could scan +/−10 degrees or more. Asanother example, it is noted that the scan speed of the Baltic 617 is 10Hz. Many ophthalmic OCT systems have B-scan rates of 30 to 40 Hz andthere are research systems running well over 100 Hz.

Single Steerable Lens

As noted, the steerable scan range of the liquid lens may increase overtime. Alternately, there may be OCT applications where a smaller scanrange is sufficient. In either of these cases, a wand scanner can beprovided with a single steerable lens instead of multiple lenses. FIG. 7illustrates a diagram of an example of a wand scanner in which a singlesteerable lens 700 is used in conjunction with a collimating lens 602 toscan the beam across the sample in accordance with embodiments of thepresent subject matter.

Wand Scanner with Additional Lenses for More Telecentric Scanning

Embodiments described hereinabove may have a curved focal plane on thesample since the liquid lens area both focusing and steering the beam.The focal length of the liquid lenses may be adjusted as they scan toflatten out the focal plane. Alternately, it may be beneficial to add alens or group of lenses between the liquid lens(es) and the sample tohelp flatten the focal plane and increase the telecentric nature of thescan.

FIG. 8 illustrates a diagram of a wand scanner in which a lens is addednear the sample to help collimated the scanning beam as it moves acrossthe sample in accordance with embodiment of the present subject matter.Particularly, FIG. 8 shows implementation of the wand scanner 106 withan additional lens set 800 of more telecentric sample scanning. The lensset 800 may be a single lens or a group of lenses. The lens set 800 maybe close to the sample or somewhere else in between the sample and theliquid lens.

FIG. 9 illustrates a diagram of an example wand scanner in which thepolarization matching occurs in the OCT engine, via manual paddles,motorized paddles or other method. Referring to FIG. 9, the scanner 106includes a circuit board for controlling the liquid lenses 606 insidethe scanner 106. The steerable liquid lenses 606 may be driven by analogelectrical signals which may be impacted by noise in cables. By placed adigital to an analog circuit board 800 in the scanner 106, thecommunication from the OCT engine to the scanner can be digital, whichis much less sensitive to noise, and the analog drive signals can begenerated in the wand scanner close to the liquid lenses.

FIG. 10 illustrates a diagram of an example wand scanner 106 in which acamera 1000 has been added to the scanner to allow visual imaging of thesample while OCT images are being taken. Referring to FIG. 10, thescanner 106 includes a wavelength beamsplitter 1002 that is placed theOCT optical path; this beamsplitter may pass wavelengths longer than˜700 nm and reflect wavelengths shorter than ˜700 nm. The visiblewavelengths from ˜400 to ˜700 nm can be sent to the camera 1000 toprovide a visual image of the sample. Suitable optics may be placed inbeampath to the camera 1000 to focus the image onto the camera 1000. Theliquid lens 606 can be used to allow adjustment of the focus. The liquidlens 606 may be a steerable liquid lens to allow adjustment of the focusand pointing of the imaging area. It may be necessary to illuminate thesample so that there is sufficient light to capture a visual image. Thismay be needed in the case of retinal imaging where there is very littleambient light available for imaging. In an example of illuminating thesample, crossed polarizers 1004 and a polarizing or non-polarizingbeamsplitter 1006 may be included and configured for minimizing glarefrom optics or other areas. The glare from optics may be polarized thesample as the illumination light, while the retina and some othersamples tend to depolarize the illuminated light when it is scatteredback. The crossed polarizers 1004 may pass some light from the retina orother sample while suppressing most of the light scattered from opticsor other areas.

Low Cost Spectrometer

In spectral domain OCT systems, the spectrometer can be the mostexpensive and sophisticated component. There are two critical componentsin the spectrometer, the line scan camera and the diffraction grating.In addition, the collimating and focusing optics and the mechanicalpackage must be well designed to maintain optical alignment over timeand temperature variations.

FIGS. 11 and 12 illustrate a cross-sectional perspective view and across-sectional top view, respectively, of the interior of an examplespectrometer 1100 in accordance with embodiments of the present subjectmatter. Referring to FIG. 11, the spectrometer 1100 may use a singleoff-axis parabolic mirror 1102. Particularly, the spectrometer 1100 mayinclude and be configured to use an off axis parabolic reflector 1104for both collimation and focusing of the light. The increasedavailability and reduced cost of single-point diamond turning hasreduced the cost of parabolic reflectors, they are now less than $100when purchased in small volumes (˜10). A parabolic reflector cansignificantly reduce the chromatic and spherical aberrations that arisein lens systems with spherical surfaces. By using a 90 degree off-axisparabolic reflector 1104 to receive light from the entrance fiber of thespectrometer, the collimated beam comes out at a right angle, hits thegrating and is diffracted back to the parabolic mirror. The parabolicreflector 1104 can focus the diffracted light to a horizontal line thatsits below the input fiber, where the line scan array detector islocated.

Referring to FIG. 12, the figure shows the ray trace of the light pathin one example of the low cost spectrometer using a single parabolicmirror. Light enters on the left via the input ferrule 1200 in themiddle of a printed circuit board 1202 and is collimated by the 90degree off-axis parabolic mirror. Light is diffracted by the reflectivegrating and comes back to the off-axis parabolic mirror 1204 where is itthen focused on the line scan array that is above or below the inputferrule. The input ferrule 1200 may be pointed slightly up or down sothat the input light is slightly off axis relative to the grating or thegrating itself may be tilted. The figure depicts the ray trace showinglight path from input fiber to an off-axis parabolic reflector 1206 andthen to a diffraction grating set 1208 at an angle of 30° relative tothe incident angle of the light. Diffracted light is collected byoff-axis parabolic and focuses onto line scan array below the inputfiber. Off-axis parabolic mirrors with other angles may be used.Commonly available versions include 30 degrees, 45 degrees, and 60degrees; additional angles are possible.

The focal length of a parabolic mirror 1210 and the line spacing of thegrating will set the dispersion of the light across the line scan array.A focal length of approximately 100 millimeters and a grating with aline density of 1200 lines/millimeter can be configured into thespectrometer 1100. As an example, both of these optical elements areavailable from of the shelf sources such as Edmund Optics. The parabolicreflector 1206 can produce a small degree of coma for the collimatedlight that comes back from the grating which is angled relative to thecentral ray. Since one version of the array is only 8 millimeters long,the angular range over the 100 millimeter focal length is +/−2.3°, whichgenerates a coma that is on the order of the pixel size of 8 microns.For this embodiment, 840 nm may be used as the central wavelength, giventhe availability of low cost SLD's in this spectral range. In SD-OCT,there is generally a trade-off between axial resolution and imagingdepth of the OCT system. Greater axial resolution means less imagingdepth due to the finite number of detection pixels. As a reasonableapproximation, the axial resolution times half the number of pixels inthe line scan array gives the imaging depth. This assumes that the lightis dispersed fully across the wavelength range of the spectrometer. Thelow cost SLDs at 840 nm have a 3 dB bandwidth of ˜45 nm. This can givean axial resolution of ˜7 microns in air or 5 microns in tissue(resolution_(tissue)=resolution_(air)×n_(tissue)). The range of thespectrometer will be ˜60 nm which give a per pixel resolution of 0.06 nmand an imaging depth of 3 millimeters in air or 2.2 millimeters intissue.

For the line scan array, a suitable CCD or CMOS array may be used, suchas the ELIS1024 (Enhanced Line-Scan Image Sensor 1024-pixel) fromDynamax Imaging. This array has 1024 pixels that are each 7.8 micronswide by 125 microns tall. The taller pixels can greatly relax themechanical tolerance for aligning the spectrometer, which cansignificantly reduce cost in manufacturing and assembly. The maximumreadout rate for the array is 30.0 MHz. A suitable circuit board may beused to interface with the linescan array. The board may have a 12 bitA/D converter, an FPGA for easy configuration, and a USB 2.0 interfacechip for communication. The data rate may be limited by the USB 2.0interface, but it can be expected to reach 20 Megasamples per second at12 bit resolution, which corresponds to an A-scan rate of 20,000 persecond, or up to 40 B-scan frames per second. Another suitable array isthe Orion from Awaiba which comes in the 2K pixel version as well as a1K and 4K pixel version.

In accordance with embodiments, the reflection grating may be replacedwith a transmission grating and a mirror. By double passing atransmission grating, double the dispersion may be obtained, so forexample instead of 1200 lp/mm reflection grating, a 600 lp/mm gratingmay be used. This may be advantageous in cases where the 600 lp/mmgrating plus mirror are cheaper than a 1200 lp/mm grating. Transmissiongratings may also have better diffraction over both polarizations whichmay be advantageous. Also, lower line density gratings may have betterspectral response over a wider wavelength range which may beadvantageous.

Low Cost Spectrometer with Loop Design

In accordance with embodiments, a spectrometer using a single off-axisparabolic mirror suffers from coma for light that does not come in onaxis. For the collimating side, the optics are essentially perfect, butthe light coming back from the diffraction grating is now spread over arange of angles corresponding to the different wavelengths. These offaxis wavelengths can suffer from coma with larger angles experiencingmore coma. For the case of a fairly small linescan array with arelatively narrow range of wavelengths, the coma may be small enoughthat the overall spectrometer performance is still acceptable.

For cases where the linescan array is larger and/or the wavelength rangeis larger, it may be advantageous to use a design where the off-axisparabolic mirror is only used for the collimating side of thespectrometer. For example, the linescan array used in the previousdesign had 1024 pixels at ˜8 microns each, so the entire length of thearray is ˜8 millimeter. More typical for spectral domain OCT systems arelinescan arrays with 2048 pixels or 4096 pixels. New arrays have pixelcounts up to 8192 or larger. Pixel sizes may be 7 microns, 10 microns,14 microns or some other size. A typical high resolution spectrometermay have 4096 pixels with a width of 10 microns each for a total lengthof 40 millimeters, which is 5 times as long as the linescan arraydescribed previously.

Likewise the spectral range for spectral domain OCT spectrometers istypically larger than the 60 nm described previously with 80 nm typicalfor retinal OCT systems with other systems having ranges up to 300nanometers or more. These larger wavelength ranges may result in largerdiffraction angle ranges since the diffraction angle range isproportional to wavelength ranges for a given line spacing of thediffraction grating.

FIG. 13 illustrates a schematic view of another example spectrometerwith loop design in accordance with embodiments of the present subjectmatter. In this example, an off-axis parabolic mirror 1300 is used tocollimate the light coming from the input aperture or fiber. After thecollimating mirror, the light is reflected off of a mirror and thendiffracted by a transmission diffraction grating. Once diffraction alens group 1302 gathers and focuses the light onto a linescan array or acamera 1304. Since the off-axis parabolic mirror has very lightchromatic or spherical aberrations, the light incident on thediffraction grating 1306 can be very well collimated. The diffractiongrating 1306 and the focusing lens group 1302 may introduce someaberrations but the overall system should perform better than aspectrometer where the collimation is done by a lens group instead ofthe off-axis parabolic mirror. Furthermore the loop architecture hasbetter mechanical stability and may be less sensitive to temperaturevariation. When combined with a tall pixel or area scan area, thesespectrometers are more tolerant to changes in environment conditionscompared to non-loop designs and spectrometers using short pixellinescan arrays.

The use of a mirror and then a grating allows the line spacing of thegrating to be changed and only impact the mirror angle. The off-axisparabolic mirror and the focusing lens group can then remain the samefor a wide range of spectrometer center wavelengths and wavelengthranges. For example in a wide bandwidth system that covers 300 nm, theline spacing of the grating may be fairly low, such at 600 lines permillimeter. In this case the incident angle may be close to 14 degrees(relative to the normal). The mirror may then be set to reflect thecollimated light to the grating at a 14 degree angle of incidence. Ifthe wavelength range is now reduced to 80 nm, the grating line spacingmay need to be closer to 1800 lines per millimeter with a grating angleof incidence of about 46 degrees. The mirror would now be set at closeto 45 degrees so that the grating angle of incidence is correct. Forboth of these cases, the other optics and the linescan array may beunchanged.

For these example of low cost spectrometers the center wavelengths andwavelength ranges discussed are just examples and can be changed asneeded for the particular OCT system and application. Availability oflight sources and sample response to various wavelengths often set thewavelength range used. For example retinal OCT is typically done withwavelengths below 900 nm, since longer wavelengths are attenuated by thewater in the eye. Other common wavelength ranges that may be usedinclude 1000 nm to 1100 nm, around 1310 nm, and around 1550 nm.

Low Cost OCT Based on Fiber Bundle and Imaging Spectrometer

In accordance with embodiments, a low cost OCT system may have thescanner removed completely and instead collect multiple A-scans inparallel using a fiber bundle. Instead of a single point beingilluminated on the sample, a line is illuminated and then imaged ontothe face of a fiber bundle. The bundle relays the light back to the OCTengine. This can be implemented using glass fiber bundles, but they arestill relatively expensive. One solution is to provide a fiber bundlemade of single mode optical fibers. As they become available, they canbe used in this design. Current glass fiber bundles are typicallymulti-mode. If these have a step index between the core and thecladding, they may still be used. In particular, there are step indexfiber bundles where there is a difference in the optical pathlengthbetween the first (or fundamental mode) and the higher order modesthrough the fiber core. Provided the pathlength difference issufficient, the OCT image can be generated by the interference of thefundamental mode and the reference arm light and the light from thehigher order modes may either be rejected by the entrance slit to thespectrometer or show up as DC signal in the spectrum or show up as asecondary image offset in depth by the difference in the pathlengths.Typically, the difference in pathlengths may be larger than the imagingrange of the spectrometer, but there are cases, such as the tissuedetection application described later, where the pathlength differencedoes not need to be greater than the imaging range since a secondaryimage is not necessarily an issue if the user is just trying to detect asurface or see just into the surface by a distance that is less than thedifference in the optical pathlengths between the fundamental mode andthe next higher order mode.

Alternately, plastic fiber optic bundles may be utilized. Some standardconfigurations have 7,400 cores and 13,000 and range in size from 0.5millimeters to 2.0 millimeters. Insertion loss ranges from 0.5 to 1.5 dBper meter. The cores are approximately 25 microns in size, so they aremulti-mode over the wavelength ranges of interest (400 nm to ˜2.0microns). As these fiber bundles improve the insertion loss can comedown and may ultimately approach glass fiber which can be a fraction ofa dB per kilometer. The number of cores may also increase and the coresize may decrease. This may increase the image size and decrease themulti-mode contributions of the fiber core.

There are multiple ways to implement the OCT engine for this embodiment;like previous Fourier domain OCT they broadly fall into spectral domainand swept source. FIG. 14 illustrates a perspective, cross-section andexploded view of another example spectrometer plus input optics for aspectral domain implementation of a fiber bundle OCT system inaccordance with embodiments of the present subject matter. Fiber bundles1400 may come in from the right. A lens set 1402 can image the bundleonto the entrance slit 1404 of the spectrometer, through the free spacesplitter 1406 where the reference arm light 1408 comes in. Once insidethe spectrometer, there is a collimating lens set 1410, a transmissiondiffraction grating 1412, a focusing lens set 1414 and an area scanarray 1416, shown here on a printed circuit board.

A system block diagram is shown in FIG. 15, which illustrates a blockdiagram for a version of the fiber bundle OCT system with the referencearm in the OCT engine. A broadband light source 1500 may generate lightand a fiber splitter 1502 may split the light into a sample path and areference path. The sample path may go out through a beam splitter 1504,the fiber bundles 1506, a lens group 1508, and onto the sample.Scattered light comes back through the lens group 1508, the imagingfiber bundle 1510 and to the beam splitter 1504. This is then mixed orinterfered with the light from the reference arm and imaged onto theentrance slit of the imaging spectrometer which is shown in furtherdetail herein. Output from the spectrometer goes to the computer via acommunication path such as USB 2.0, USB 3.0 or other means. Here, thebroadband light source 1500 is used in a Mach-Zehnder interferometerconfiguration. The light from the source is split with part going to thesample and part remaining in the OCT engine. The light on the sampleilluminates a line with scattered light then imaged onto the face of thefiber bundle 1510. A row (or several rows) of fibers near the middle ofthe bundle then carry the light from different spatial locations back tothe OCT engine. Here, it interferes with light that remained in the OCTengine via free space beam splitter. From here, the light is incident onthe slit of an imaging spectrometer. In this view, the spatial locationsfrom the sample are mapped in the vertical direction and the wavelengthrange is mapped horizontally across the area scan array. Implementationshow uses a lens group 1512 for collimation, a transmission diffractiongrating and a focusing lens group. The detector may be any array scanarray that may be, for example, a CMOS area scan array.

Alternately, these designs may be implemented using a swept sourceapproach. Here the light source is a laser that sweeps its wavelength intime and on the detection end the spectrometer is replaced by a linescanarray. The linescan array will make an acquisition for each wavelengththat is desired during a sweep. The time sequence of linescans thenprovides the same information as the 2D area scan array in the spectraldomain implementation.

Since the light source 1500 is spread across a line instead of focusedat a single point, the optical power in each A-scan may be lower. In thereference arm path, this may not be an issue since there is typicallyexcess optical power in the reference arm. The lower optical power mayresult in a lower signal to noise for the OCT image. This can beameliorated to some degree by integrating for a longer time since anentire B-scan is being acquired at once. For example, an OCT system mayacquire 20 B-scans in a second with 512 A-scans per B-scan. This cangive an overall linescan rate of 10,000 per second or an integrationtime per A-scan of about 100 microseconds. In the fiber bundle, anapproach to reach 20 B-scans per second may require 20 integrations ofthe array scan array so the integration time can be as large as 50milliseconds, thereby making up for the lower power in a given A-scan.There is a limit to how effective this can be for samples that may bemoving. Any motion can decrease the fringe contrast in theinterferometer. In particular, biological systems move on time scalesthat start to wash out fringes for integration times longer than a fewhundred microseconds. For static samples, this may be less of an issue.

FIG. 16 illustrates a block diagram for an example fiber bundle OCTsystem with the reference arm in fiber probe in accordance withembodiments of the present subject matter. Referring to FIG. 16, lightfrom the light source 1500 is imaged onto the fiber bundle 1510. In thefiber probe tip 1600, there is a layer 1602 that either reflects orscatters light and acts as the reference arm. The rest of the light maybe incident on the sample and may scatter back. The interfered light isthen imaged back onto the fiber bundle 1510 and transmitted to the OCTengine where another lens set images it onto the entrance slit of theimaging spectrometer as before. FIG. 16 shows another implementationwhere the interferometry all occurs in the in the tip 1600 of the fiberprobe. Light from the broadband light source 1500 is imaged onto part ofthe fiber bundle 1510 and then transmitted to the distal end of thebundle. A lens group 1508 may focus the light onto the sample and onto ascattering or reflective layer 1602. The focus can be at or in thesample so the scattering layer may be out of focus. The scattering layercan be configured to scatter enough light to fill the dynamic range ofthe area scan array in the imaging spectrometer in the neededintegration time. The scattered light may be minimized as much aspossible to increase the amount of light on the sample and because thelight scattered from the sample may have to pass through the scatteringlayer a second time and the signal can be reduced in proportion to thestrength of scattering in the scattering layer. The scattering layer maybe configured to scatter light over a narrow enough range to illuminatethe fiber bundle even as the conditions in the fiber probe change do thechanges in the orientation of the bundle or manufacturing variations inthe fiber bundle or the probe tip optics.

It is noted that in all places where a lens group or set is mentioned,there are many ways to implement this function. The lens group or setmay be one or more elements. These elements may be refractive (i.e.,transmission) optics or reflective optics such as the off-axis parabolicmirror described earlier. In some case, it may be useful to use one ormore GRIN lenses, particularly in the probe tip. Aspheric lenses mayalso be useful in some locations.

FIGS. 17A-17E illustrate diagrams of example probe tip geometries andoptics in accordance with embodiments of the present subject matter.These different implementations can provide different scan patterns onthe sample and may be driven by the particular application. Moreparticularly, FIG. 17A shows a lens group (e.g., GRIN lens(es)) thatprovide a straight line image on the sample, directly ahead of the tipof the fiber probe. FIG. 17B shows a fisheye lens (or lenses) that givesa semicircle line on the sample and may see outside of the lateral rangeof the fiber probe. FIG. 17C shows a mirrored prism that directs theimaging line out the side of the bundle. The configuration of FIG. 17Cmay be used with rotation and/or pullback to generate a scan of a tube,blood vessel, or other tubelike structure. FIG. 17D shows a partialfisheye images both straight ahead and out to one side. Again may beused with rotation and/or pullback to generate a larger scan. FIG. 17Eshows an axicon can be used to send the light out it multipledirections. The axicon may have a mirrored surface (silver, gold,aluminum or other). Depending on the position of the center of theaxicon relative to the center of the imaging line on the fiber bundle,the imaging line on the sample may be two lines on either side of thefiber bundle or an arc across the sample.

There are applications where the low cost and small size may be moreimportant than the loss of signal to noise. For example there areapplications where OCT may be used more as a ranging modality than animaging modality. One of these in intubating patients—by using a fiberbundle that could be passed through the intubation tube it would bepossible to see where the tube is relative to important landmarks in theairway. In this case imaging into the tissue as is often done with OCTis not as important as seeing where the surface of the tissue isrelative to the intubation tube. The tissue surface will generate a verystrong OCT signal even if the system signal to noise is not very good.This may be useful for such things as insuring the tube is in the airwayand not the esophagus, avoiding the vocal cords, and insuring that thetube is not in the single bronchial tube, but instead stops in theairway before the bronchial tubes branch off.

The various techniques described herein may be implemented with hardwareor software or, where appropriate, with a combination of both. Thus, themethods and apparatus of the disclosed embodiments, or certain aspectsor portions thereof, may take the form of program code (i.e.,instructions) embodied in tangible media, such as floppy diskettes,CD-ROMs, hard drives, or any other machine-readable storage medium,wherein, when the program code is loaded into and executed by a machine,such as a computer, the machine becomes an apparatus for practicing thepresently disclosed subject matter. In the case of program codeexecution on programmable computers, the computer will generally includea processor, a storage medium readable by the processor (includingvolatile and nonvolatile memory and/or storage elements), at least oneinput device and at least one output device. One or more programs may beimplemented in a high level procedural or object oriented programminglanguage to communicate with a computer system. However, the program(s)can be implemented in assembly or machine language, if desired. In anycase, the language may be a compiled or interpreted language, andcombined with hardware implementations.

The described methods and apparatus may also be embodied in the form ofprogram code that is transmitted over some transmission medium, such asover electrical wiring or cabling, through fiber optics, or via anyother form of transmission, wherein, when the program code is receivedand loaded into and executed by a machine, such as an EPROM, a gatearray, a programmable logic device (PLD), a client computer, a videorecorder or the like, the machine becomes an apparatus for practicingthe presently disclosed subject matter. When implemented on ageneral-purpose processor, the program code combines with the processorto provide a unique apparatus that operates to perform the processing ofthe presently disclosed subject matter.

Features from one embodiment or aspect may be combined with featuresfrom any other embodiment or aspect in any appropriate combination. Forexample, any individual or collective features of method aspects orembodiments may be applied to apparatus, system, product, or componentaspects of embodiments and vice versa.

While the embodiments have been described in connection with the variousembodiments of the various figures, it is to be understood that othersimilar embodiments may be used or modifications and additions may bemade to the described embodiment for performing the same functionwithout deviating therefrom. Therefore, the disclosed embodiments shouldnot be limited to any single embodiment, but rather should be construedin breadth and scope in accordance with the appended claims.

What is claimed:
 1. A high-resolution spectrometer comprising: an inputaperture; an off-axis parabolic mirror configured to receive light, fromthe input aperture, along a first direction, and configured to collimatethe light and reflect the light along a second direction; a fold mirrorconfigured to turn the light from the second direction to a thirddirection; a diffraction grating configured to spectrally disperse thelight and to turn the light to a fourth direction that is about 180degrees from the second direction; a lens set configured to receive thedispersed light to cause it to be focused at a detection plane; and asensor positioned at the detection plane and being configured to convertintensity of the detected light to electrical signals, wherein the foldmirror and the diffraction grating are configured such that the lightpath from the diffraction grating to the sensor crosses the light pathfrom the input aperture to the off-axis parabolic mirror.
 2. Thespectrometer of claim 1, wherein the input aperture comprises an outputof an optical fiber.
 3. The spectrometer of claim 1, wherein lightincident on the sensor is nearly perpendicular to the sensor.
 4. Thespectrometer of claim 1, wherein the sensor is a tall pixel line scanarray.
 5. The spectrometer of claim 4, wherein the line scan array iscomplementary metal oxide semiconductor (CMOS) based.
 6. Thespectrometer of claim 4, wherein the line scan array is Indium GalliumArsenide (InGaAs) based.
 7. The spectrometer of claim 4, wherein theline scan array is configured for 1310 nm light.
 8. The spectrometer ofclaim 4, wherein the off-axis parabolic mirror is a 90-degree mirror. 9.The spectrometer of claim 4, wherein the diffraction grating is atransmission grating.
 10. A high-resolution spectrometer comprising: aninput aperture; an off-axis parabolic mirror configured to receive lightfrom the input aperture, along a first direction, and configured tocollimate the light and reflect the light; a transmission diffractiongrating configured to spectrally disperse the light; a mirror configuredto reflect the dispersed light back through the grating and onto theoff-axis parabolic mirror such that the parabolic mirror focuses thedispersed light along a second direction towards a detection plane thatcorresponds to the same plane as the input aperture; and a sensorpositioned in the detection plane relative to the input aperture toconvert the light intensity to electrical signals, wherein the firstdirection and the second direction are in a plane that is orthogonal tothe plane defined by the midpoint between the input aperture and thesensor, the middle of the off-axis parabolic mirror and the middle ofthe diffraction grating.
 11. The spectrometer of claim 10, wherein theinput aperture comprises an output of an optical fiber.
 12. Thespectrometer of claim 10, wherein the sensor is a line scan array. 13.The spectrometer of claim 12 wherein the line scan array iscomplementary metal oxide semiconductor (CMOS) based.
 14. Thespectrometer of claim 10, wherein the line scan array has tall pixels.15. A high-resolution spectrometer comprising: an input aperture; anoff-axis parabolic mirror configured to receive light from the inputaperture, along a first direction, and to collimate the light andreflect the light; a reflection diffraction grating configured todisperse the light and to reflect the light back onto the off-axisparabolic mirror toward a detection plane which corresponds to the sameplane as the input aperture, wherein the off-axis parabolic mirrordirects the light along a second direction; and a sensor positioned inthe detection plane relative to the input aperture to convert the lightintensity to electrical signals, wherein the first direction and thesecond direction are in a plane that is orthogonal to the plane definedby the midpoint between the input aperture and the sensor, the middle ofthe off-axis parabolic mirror and the middle of the diffraction grating.16. The spectrometer of claim 15, wherein the input aperture comprisesan output of an optical fiber.